Dynamic modeling and solubility studies of sour gases during sweetening process of natural gas

Abstract

Sour gas components (mostly H2S and CO2) are to be removed from natural gas in a sweetening process. Analysis of sweetening of natural gas has been carried out by formulating steady state and transient rate-based models which have been developed based on film theory and surface renewal theory for the two phases (gas and liquid) in the industrial absorber considering vapor liquid equilibria. The analytical equations representing the model were simulated and the profiles of sour components were compared with industrial data for validation. The model considers kinetics of reaction between solvent methyl-di-ethanolamine (MDEA) which enters the column from top and feed sour gas from bottom containing H2S and CO2. The concentration profiles of acid gas components are obtained for both the phases. Transient model representing temperature dynamics has been validated using industrial data with a mean square error of 0.4475. The removal efficiency for sour-gas is 95%. A quadratic regression model based on statistical data has also been formulated to predict thermodynamic properties concerning solubility of sour gas components in the solvent under different temperatures and pressures. Optimization of model reveals that separation of H2S is favoured at T = 318.46 K, P = 707.12 kPa and concentration = 4.42 mol/lit while optimal operating points for CO2 are T = 338.23 K, P = 3535.69 kPa and concentration = 4.43 mol/lit. It can be observed that compared to H2S dynamics, CO2 dynamics is sluggish. The comprehensive form of the rate-based model can be used for formulating control strategies.